1. Technical Field
The invention is directed to a circulating current sensing device and more particularly to a circulating current sensing device for sensing circulating currents in power amplifiers.
2. Related Art
Paralleling of amplifiers (fast four quadrant DC to AC power converters) has been done for some time. More recently, however, amplifier designs are changing from linear to switch-mode technology. In addition, the environment in which amplifiers operate is continuing to demand larger amounts of power. When paralleled amplifiers do not share current, costly inefficiencies arise.
At first paralleling of amplifiers was done using simple passive ballasting. Linear amplifiers had wide bandwidth and fairly small phase errors that led to substantial conformity of gain and phase characteristics. High frequency circulating currents were reduced by using a highly coupled center tapped inductor whose center tap joined to the loads and whose ends attached to an amplifier output. If the amplifiers are delivering equal currents, such an inductor will store no net energy and thus no signal voltage will be lost to inductance. It is important not to loose signal voltage as the costs of generating large amounts of power are also large.
When the demands on the ballast resistors grew to more than 250 Watts of dissipation, negative current feedback was used to synthesize an effective amplifier output resistance (lossless). This constituted a second and improved generation of paralleling design.
With the advent of high efficiency switch-mode amplifiers additional issues have arisen. Output currents are typically larger and the gain and phase characteristics are now much looser in tolerance, potentially making current sharing more difficult.
One of the uses for the subject paralleled amplifiers is in the medical industry, for use with magnetic resonance imaging (MRI), where the load on the system is the gradient coil of the MRI device. This environment is relatively hostile for gradient signal processing, because the MRI device has large amounts of peak RF power ( less than =20KW) supplied to coils that are immediately inside the gradient coils. With such intimate coupling, it is necessary to place low-pass filters in the feed lines to the gradient coils to contain the RF currents. These filters tend to aggravate an already bad situation for establishing wide bandwidth negative current feedback. Large phase response lags within the amplifiers and distributed capacitances in the gradient coils already have limited the amount of feedback that can be used to control the system. Any controls added to effect current sharing cannot corrupt the output signal as there may be insufficient feedback to correct any significant injected non-linear errors. Therefore some of the methods practiced by the DC to DC converter industry for current sharing are not applicable here.
What is desired is a lossless mechanism for sensing circulating (unbalanced) currents caused by mismatched parallel power amplifiers and introduction of output corrections in such a manner as to not influence the net output available to the load. This implies that the entire methodology is lossless and also has no net output inductance added to the load circuit.
The invention provides an amplifier system of two or more (N) parallel power amplifiers that use current sensors to directly measure the circulating currents and, by use of negative feedback, regulate the circulating current to zero. The amplifier system may use passive magnetic device(s) to facilitate current sharing, where the passive magnetic device(s) are each designed to store no magnetic energy when under balanced excitation.
A generalized method for balancing paralleled power amplifiers is disclosed wherein (N) power amplifiers, are paralleled and have current sensors positioned so as to form a differencing equation for the circulating current. The difference in circulating current may be used as feedback to the paralleled power amplifiers to force the circulating current to zero. The current sensors may be current transforming transducers, where (Nxe2x88x921) transducers are included in the system. The feedback from the (Nxe2x88x921) transducers is distributed to summing amplifiers. The summing amplifiers may balance the power amplifiers according to the gain distribution of the summing amplifiers. The amplifier system may also include passive magnetic device(s) to facilitate current sharing. The passive magnetic device(s) are generally inductors which are designed to store no magnetic energy when under balanced excitation.
A power amplifier system that includes two or more power amplifiers with outputs capable of being paralleled may also use a circulating current sensing device. The circulating current sensing device may include the functionality of both a current sensor(s) and a passive magnetic device(s) in one device to minimize unbalanced operation of the paralleled power amplifiers. The circulating current sensing device may include an inductive current transformer and a preamplifier.
The inductive current transformer may have a core that is a passive magnetic device capable of passive control to minimize unbalanced amplified output power from the paralleled power amplifiers. The core may include a slot with at least one sensor positioned in the slot. The sensor may sense magnetic flux resulting from the circulating current and provide signals indicative of circulating current to the preamplifier. The preamplifier may process the signals to provide a negative feedback signal that is an error correcting signal. The negative feedback signal may be used for active control of unbalanced amplified output power from the paralleled power amplifiers. Accordingly, unbalanced amplified output power of paralleled power amplifiers may be minimized by the combination of the active control and the passive control.
Sensing of the circulating currents may be performed by positioning the circulating current sensing device to measure the amplified output power from each of the power amplifiers. Alternatively, the circulating current sensing device may be positioned to directly measure the circulating current from positive and negative output stage power of a positive and negative output stage included in each of the power amplifiers.
Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims.